Reduction of soluble iron and reductive dissolution of ferric iron-containing minerals by moderately thermophilic iron-oxidizing bacteria

Abstract

Five moderately thermophilic iron-oxidizing bacteria, including representative strains of the three classified species (Sulfobacillus thermosulfidooxidans, Sulfobacillus acidophilus, and Acidimicrobium ferrooxidans), were shown to be capable of reducing ferric iron to ferrous iron when they were grown under oxygen limitation conditions. Iron reduction was most readily observed when the isolates were grown as mixotrophs or heterotrophs with glycerol as an electron donor; in addition, some strains were able to couple the oxidation of tetrathionate to the reduction of ferric iron. Cycling of iron between the ferrous and ferric states was observed during batch culture growth in unshaken flasks incubated under aerobic conditions, although the patterns of oxidoreduction of iron varied in different species of iron-oxidizing moderate thermophiles and in strains of a single species (S. acidophilus). All three bacterial species were able to grow anaerobically with ferric iron as a sole electron acceptor; the growth yields correlated with the amount of ferric iron reduced when the isolates were grown in the absence of oxygen. One of the moderate thermophiles (identified as a strain of S. acidophilus) was able to bring about the reductive dissolution of three ferric iron-containing minerals (ferric hydroxide, jarosite, and goethite) when it was grown under restricted aeration conditions with glycerol as a carbon and energy source. The significance of iron reduction by moderately thermophilic iron oxidizers in both environmental and applied contexts is discussed.

FIG. 1

Oxidoreduction of iron by strains of moderately thermophilic, acidophilic bacteria grown in nonshaken cultures (100 ml of medium/250-ml conical flask) containing 25 mM ferrous sulfate, 10 mM glycerol, and 0.02% (wt/vol) yeast extract (pH 2.0) and incubated at 45°C. Symbols: •, S. acidophilus ALV; ▪, S. acidophilus THWX; ▴, S. acidophilus YTF1; ○, S. thermosulfidooxidans TH1; ▿, A. ferrooxidans TH3; +, cell-free control.

FIG. 2

Reduction of ferric iron by strains of moderately thermophilic, acidophilic bacteria grown in anaerobic cultures containing 20 mM ferric sulfate, 10 mM glycerol, and 0.02% (wt/vol) yeast extract (pH 2.0) and incubated at 45°C. Symbols: •, S. acidophilus ALV; ▪, S. acidophilus THWX; ▴, S. acidophilus YTF1; ○, S. thermosulfidooxidans TH1; ▿, A. ferrooxidans TH3; +, cell-free control.

FIG. 3

Reduction of ferric iron by strains of moderately thermophilic, acidophilic bacteria grown in anaerobic cultures containing 20 mM ferric sulfate and 5 mM potassium tetrathionate (pH 2.0) and incubated at 45°C. Symbols: •, S. acidophilus ALV; ▪, S. acidophilus THWX; ▴, S. acidophilus YTF1; ○, S. thermosulfidooxidans TH1; ▿, A. ferrooxidans TH3; +, cell-free control.

FIG. 4

Relationship between ferric iron reduction and biomass yields, as determined by optical densities of resuspended bacteria (a) and counting of moderately thermophilic iron-oxidizing bacterial cells grown under anaerobic conditions (b). Symbols: ○ and •, S. acidophilus YTF1; ▿ and ▾, S. thermosulfidooxidans TH1; ▵ and ▴, A. ferrooxidans TH3. The linear correlation coefficients (r values) are shown. OD (600 nm), optical density at 600 nm.

FIG. 5

Dissolution of amorphous ferric hydroxide (a), jarosite (b), and goethite (c) by moderately thermophilic, acidophilic bacteria. Cultures were grown under microaerobic conditions in media (pH 2.0) containing 10 mM glycerol and 0.02% (wt/vol) yeast extract; each of the media also contained one of the ferric iron-containing minerals at a concentration of 0.1%, and the media were incubated at 45°C. Symbols: •, S. acidophilus ALV; ▴, S. acidophilus YTF1; ○, S. thermosulfidooxidans TH1; ▿, A. ferrooxidans TH3; +, cell-free control.

FIG. 6

Speciation of soluble iron in cultures of S. acidophilus YTF1 grown under microaerobic conditions in media (pH 2.0) containing 10 mM glycerol and 0.02% (wt/vol) yeast extract and supplemented with 0.1% (wt/vol) amorphous ferric hydroxide (a), 0.1% (wt/vol) jarosite (b), or 0.1% (wt/vol) goethite (c). Symbols: •, total iron; ▵, ferrous iron; ○, ferric iron.